Hitherto, various methods have been proposed and practically employed for overwrap-packaging an object with a plastic film.
For instance, packaging methods of the type which seals three sides of a package include one adapted to heat-seal all the three sides of a package by melt-cut sealing or band sealing (heat-sealing using a heating bar, heating continuous belt or heating disk edge to form a band-like sealed portion), one adapted to fin-heat-seal one of the three sides of a package and to melt-cut-seal or band-seal the two remaining sides thereof, and one adapted to electrostatic-seal one side of a package in a lap seal fashion and to melt-cut-seal or band-seal the two remaining sides thereof.
In addition, there is known a packaging method using a stretched film wherein one side of a package is lap-heat-sealed and the two remaining sides thereof are folded onto the bottom surface of the package and fixed thereto by heat sealing.
Such conventional methods, however, involve a problem that where packages each having a heat-sealed portion resulting from melt-cut sealing on the bottom surface thereof are transported or displayed in a stacked fashion, the melt-cut-sealed portion on the bottom surface of an upper package is brought into close contact with the top surface of the underlying package and rubbed thereby due to vibration or the like and is hence prone to rupture.
Although rupture of such kind is hard to occur if a package is fin-heat-sealed on the bottom surface thereof, another problem arises that since the fin-heat-sealed portion of the package is considerably thick and forms a projection on the bottom surface of the package, stacking this package on an object having, for example, a soft surface would frequently leave a mark of the sealed portion on the top surface of the underlying object.
With the package which is electrostatic-sealed on the bottom surface thereof in a lap seal fashion, the problem of the aforementioned marking will not arise because the sealed portion is thin and does not project so much. However, such a package suffers a problem that the sealed portion is likely to be unsealed since it is not fusion-bonded and a problem that if the package contains a liquid such as water or juice, such a liquid is easy to leak from the package. Further, in this case another problem is present that since the packaging material must be an electrostatically chargeable film, such a film is easy to collect dust due to static electricity and is inevitably poor in antifog properties.
Further, with the package using a stretched film wherein one side of the package is lap-heat-sealed and the two remaining sides thereof are folded onto the bottom surface of the package and fixed thereto by heat sealing, there is a drawback that the portions folded onto the bottom surface of the package are wrinkled and, hence, openings are left in the wrinkled portion to frequently permit a liquid to leak therethrough. In addition such a wrinkled portion degrades the appearance of the package.